update_to_5.0.md: About ibexa/compatibility-layer

Author: adriendupuis

docs/update_and_migration/from_4.6/update_to_5.0.md

@@ -27,6 +27,10 @@ It supports only PHP 8.3 and above.
 
 It's important to stop using deprecated PHP classes as they're removed in 5.0.
 
+The [`ibexa/compatibility-layer`](to_4.0.md#add-compatibility-layer-package) isn't supported in 5.0.
+If you use it, remove it (`composer remove ibexa/compatibility-layer`) and make the necessary changes.
+See [Ibexa DXP v4.0 deprecations and backwards compatibility breaks](ibexa_dxp_v4.0_deprecations.md) for some of the changes it was avoiding.
+
 [Rector](https://getrector.com/) and the [[= product_name_base =]] rule sets help to upgrade your code.
 
 Install [`ibexa/rector`](https://github.com/ibexa/rector) which contains rules to ensure custom code is up to date with DXP 4.6: